Write a solution for this word problem?

30 liters of a 40% acid solution is obtained by mixing a 25% solution with a 50% solution.
Write a system of equations in which one equation represents the amount of final mixture required and the other represents the percent of acid in the final mixture. Let x and y represent the amounts of the 25% and 50% solutions, respectively.

I know how to solve these type of equations, but I'm not sure how to make equations to fit this!